Continuous process for the extraction of monomers and oligomers from highly polymerized caprolactam granules



March 19, 1968 K. RYFFEL ETAL CONTINUOUS PROCESS FOR THE EXTRACTION OFMONOMERS AND OLIGOMERS FROM HIGHLY POLYMERIZED CAPROLACTAM GRANULESFiled Nov. 12. 1964 v INVENTORS KASPIR RYFFEL WA! TER ZEHIVDER Ma /Ma,

AT TORNEYS United States Patent 5 Claims. (Cl. 260 -78) ABSTRACT OF THEDISCLOSURE A process for the extraction of monomers and oligomers fromhighly polymerized poly-E-caprolactam granules and recovery for re-use.The granulated polymer containing the monomers and oligomers is causedto slide through a vertical tube and is contacted by an extractant incountercurrent.

This application is a continuation-in-part of our copending applicationSer. No. 859,528, filed Dec. 14, 1959, and now abandoned.

The invention relates to a process of refining of granulated polyamidesand, more particularly, to the removal of constituents of low molecularweight therefrom. It is especially applicable to poly-E-caprolactam,known as nylon-6.

Poly-E-caprolactam is a highly polymerized material, as is generallyknown. However, constituents of low molecular weight initially arepresent therein which adversely atfect the physical and mechanicalproperties of the nylon-6, which is used for molding, extrusion, andparticularly for the formation of synthetic fibers. The latter use is ofespecial interest in connection with the present invention, and itspreferred use therefore is directed to fiber-forming poly-E-caprolactamalbeit the use for molding resins is not to be considered excluded. Thefiber-forming grade notoriously has the highest degree ofpolymerization.

The constituents of low molecular weight mentioned are monomeric andoligorneric caprolactam, and the oligomer substantially is the dimer andthe timer of caprolactam, usually in mixture. Their removal from highlypolymerized nylon-6 has met with diiliculties in the past.

Before the granulated poly-E-caprolactam is fabricated by injectionmolding or, in the case of fiber-formers, in spinnerets, thesegranulates therefore must be subjected to a washing process to free themfrom the monomer and oligomers. This frequently is accomplished by awater wash, as for instance, by the so-cal1ed chip-washing as describedin Swiss Patent 2303271, and usually carried out by the so-called layerprocess. This process suifers from the drawback that the concentrationof the monomer and oligorner in the wash water is very low so that theirrecovery becomes very costly. The layer process has the additonaldisadvantage of non-uniformity of the residual content of monomers andoligomers in the nylon-6 thus treated.

Washing in boiling water, as taught in US. Patent 2,978,439, leads to adiscoloration of the high-polymer thus treated, so that reducing agents,e.g., hydrogen sulfite, sulfur dioxide and/0r hydrazine hydrate must beincorporated in the wash water which may lead to contamination, and eventhese additions do not preclude the discoloration, i.e., yellowing,without fail. In this highly competitive nylon market, a yellowedproduct is unsalable, besides it is an indication of oxidation. Theemployment of boiling water or steam, the latter disclosed in US.

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2,867,805, causes degradation and also yellowing of thepoly-E-caprolactam.

It now has been found that granulated high-polymeric poly-E-caprolactamcan continuously be freed from its monomeric and oligomeric constituentsby causing the granules to slide through a vertical tube incounter-current to a suitable solvent. Simultaneously, the monomers andoligomers can be concentrated in the solution obtained and can bere-used for further polymerization in that solution or after recoverytherefrom.

The process according to the invention is applicable particularly tofiber forming nylon-6 and will be described in detail with reference tothe accompanying drawing, which is an elevation of the extractiondevice.

Through a vertical tube 1 granulate 2' is poured into the treatmentchamber through a feed device 2. Extracting agent is pumped from belowinto the extraction chamber through pipe 4 and the annular passage 5 andis drawn off through the pipe 6 after absorbing the constituents of lowmolecular weight. The granulate is removed from the extraction chamberthrough a discharge device 3. Because the granulate is made to slidethrough the extraction column as a compact layer, it is possible toadjust the time of sojourn and thereby the degree of extraction asdesired. If, in addition to obtaining as complete an extraction of thelow molecular weight constituents as possible, it is desired to obtainthe aqueous solution thereof as concentrated as possible at the sametime, it is preferred, according to a further feature of the processaccording to the invention, to cause the quantity of extracting agentwhich flows through the column per unit of time at certain heights orlevels to be reduced from the bottom upwards. In order to achieve this,extracting agent is drawn off the column at various heights. One ofthese removal point-s 7 is shown in the drawing. Part of the extractingagent fed through pipe 4 thus is drawn off at this point through thepipe 8. Of course, care must be taken that the extracting agent coversthe entire layer of granulate right to the very top of the extractionchamber. This can be done, for example, by maintaining the correct ratioof the fed quantity of extracting agent to that removed at theintermediate points .of removal, i.e., care must be taken that thequantity supplied is always larger than the quantity removed at theintermediate points of removal. Owing to the fact that the quantity ofextracting agent which flows from the bottom upwards per unit of timethrough the extraction chamber and thereby through the granulatedecreases, it is possible both to enrich the extract content in theextracting agent in the upper part of the column and to extract from thegranulate with the large quantities of extracting agent in the lowerpart of the column the constituents of low molecular Weight, which arepresent only in small quantity at this point.

The extraction may also be carried out at elevated temperatures. Theextracting agent then is preheated and the apparatus is heated by aheating medium, circulating in heating jacket 9. If the extraction is tobe carried out at temperatures above the boiling point of the extractingagent, this must be done under pressure. In this case, the granulatemust be introduced through a pressure lock.

In order to make the extracting action as efiicient as possible, it isadvantageous to take care that the extracting agent either has the sametemperature over the entire height of the column or a temperatureincreasing from bottom to top thereof. It is also advantageous to keepthe ratio of tube diameter to tube length, at least 1:5, prefera-bly atabout 1:40. By means of these measures, any harmful cross-currents whichmay be produced because of differences of temperatures and differencesof density of the extracting agent can practically be eliminated. The

3 tratio of tube length to tube diameter also should not be too large,i.e., it should not exceed 100: 1.

It is clear that the optimum time of sojourn of the .granulate isdependent upon the solubility of the low molecular components in thesolvent, on the extraction temperature and also on the velocity ofdiffusion of the extracted substances from the polyamide and thereby,naturally, also on the grain size of the granulate.

The process will now be further described by the following examples.However, it should be understood that these are given merely by way ofexplanation, not of limitation, and that numerous changes may be made inthe details without departing from the spirit and the scope of theinvention as hereinafter claimed.

Example 1 illustrated the operation of an extraction process in whichthe quantity of extracting agent is not reduced from the bottom upwards.In this way, lactam solutions of low concentration are obtained. Example2 shows the extraction in which some of the solvent is drawn off thecolumn.

For reasons of economy, water preferably is employed as the extractingagent, but all solvents which dissolve the low molecular weightconstituents without harming the polymer may be used, such, as forexample, low monoalcohols. Water must not be at boiling temperatureunless under pressure. At atmospheric pressure, the maximum temperatureis 95 C. A range of substantially 90-95 C. is preferred although lowertemperatures may -be employed.

Example 1 Poly-E-caprolactam granulate in the form of chips 2.5 mm. indiameter and 2.5 mm. thick is extracted in countercurrent flow withwater at 95 C. in an extraction column of 4 meters length and 150 mm.diameter. The rate of discharge is 6 kg./h. of chips calculated on thedry weight, which corresponds to a time of sojourn of about 7 /2 hours.In the process, the content of extractable constituents decreases from10.2 to 1.3 percent by weight. Pure water is fed into the bottom of thecolumn from a leveling vessel and 9.3 l./h. lactam solution with acontent of 6.2% of extract are drawn off at the top of the column bymeans of a dosing pump.

Example 2 Poly-E-caprolactam chips having a grain size of 2.5 mm. areextracted with water at 95 C. in an extraction apparatus of 8 meterslength comprising six parallel tubes of 150 mm. diameter. The column isprovided with an intermediate tapping point for lactam solution at aheight of 2 meters from the bottom end. A total quantity of 22 kg./h. ofchips (calculated on the dry state) is drawn off in equal amounts at thebottom from all six tubes. In the process, the content of extractableconstituents is lowered from 10.5 to 0.5%. Pure water is fed into thebottom of the column from a leveling vessel, and a total quantity of 9l./h. of a 22% lactam solution are drawn off at the top of the column inequal quantities from each tube by means of a six-part pump. Through theintermediate tapping, a quantity of 16 l./h. of a 2.2% solution islikewise drawn off by means of a six-part pump. The lactam solutioncontains caprolactam in monomeric and oligomeric form.

In both examples water-white poly-E-caprolactam was recovered which hadnot in the least discolored.

We claim as our invention:

1. A continuous process for the removal of monomeric and oligomericcaprolactam from granulated highly polymerized poly-E-caprolactamincluding fiber-forming grades while preventing its yellowing and forthe concentration and recovery of said caprolactam, which comprisescountercurrently washin g a compact layer of said granules, having anaverage grain size of substantially 2.5 mm., with a liquid which is asolvent for said caprolactam but .a non-solvent for the polymer, in anextract or consisting essentially of a vertical cylindrical tube havinga ratio of inside diameter to height of at least 1:5 and having aplurality of solvent outlets at varying heights, the topmost outletbeing disposed near the top of said extractor; said washing beingcarried out by continuously feeding substantially 6 to 22 kg./h. of saidgranules into said extractor from the top and allowing the compact layerformed to slide downward while continuously feeding solvent into saidextractor at its bottom and allowing it to travel upward; continuouslydrawing off portions of said solvent, now containing monomeric andoligomeric caprolactam, at said varying heights, but at all times fullycovering said compact layer with solvent, in total amounts ofsubstantially 9 to 25 1./h. and containing said caprolactam inquantities up to 22 percent; continuously drawing off washed granules atthe bottom of said extractor; and recovering the monomers and oligomersfrom their solution.

2. A continuous process for the removal of monomeric and oligomericcaprolactam from granulated highly polymerized poly-E-caprolactamincluding fiber-forming grades while preventing its yellowing and forthe concentration and recovery of said caprolactam, which comprisescountercurrently washing a compact layer of said granules, having anaverage grain size of substantially 2.5 mm., with a liquid which is asolvent for said caprolactam but a non-solvent for the polymer, in anextract or consisting essentially of a vertical cylindrical tube havinga ratio of inside diameter to height of 1:5 to 1:40 and a ratio ofheight to inside diameter not exceeding 1:100, said extractor having aplurality of solvent outlets at varying heights, the topmost outletbeing disposed near the top of said extractor; said washing beingcarried out by continuously feeding substantially 6 to 22 kg./ h. ofsaid granules into the top of said extractor and allowing the compactlayer formed to slide downward while continuously feeding solvent intosaid extractor at its bottom and allowing it to travel upward;continuously drawing off portions of said solvent, now containingmonomeric and oligomeric caprolactam, at said varying heights but at alltimes fully covering said compact layer with said solvent, in totalamounts of substantially 9 to 25 l./h. and containing said caprolactamin quantities up to 22 percent; continuously drawing off washed granulesat the bottom of said extractor; and recovering the monomers andoligomers from their solution.

3. The process as defined in claim 2, wherein said counter-currentextraction is carried out in an extractor consisting of a plurality ofparallel vertical cylindrical tubes.

4. A continuous process for the removal of monomeric and oligomericcaprolactam from granulated highly polymerized poly-E-caprolactamincluding fiber-forming grades while preventing its yellowing and forthe concentration and recovery of said caprolactam, which comprisescountercurrently washing a compact layer of the granules, having anaverage grain size of 2.5 mm. with water below its boiling point in anextractor consisting essentially of a vertical cylindrical tube having aratio of inside diameter to height of at least 1:5 and having aplurality of water outlets at varying heights, the topmost outlet beingdisposed near the top of said extractor; said washing being carried outby continuously feeding substantially 6 to 22 kg./h. of said granules inthe extractor at its top and allowing the compact layer formed to slidedownward while continuously feeding hot water into said extractor fromits bottom and allowing it to travel upward; continuously withdrawingportions of said water, now containing monomeric and oligomericcaprolactam, at said varying heights but at all times fully coveringsaid compact layer with water, in total amounts of substantially 9 to 25l./h. and containing said caprolactam in quantities up to 22 percent;continuously drawing off washed granules at the bottom of saidextractor; and recovering the monomers and oligomers from their aqueoussolution.

5. A continuous process for the removal of monomeric and oligomericcaprolactam from granules of highly polymerized poly-E-caprolactamincluding fiber-forming grades while preventing its yellowing and forthe concentration and recovery of said caprolactam, which compriseswashing a compact layer of the granules, having an average grain size ofsubstantially 2.5 mm., with water of substantially 90-95 C. atatmospheric pressure in an eX- tractor consisting essentially of avertical cylindrical tube provided with heating means and having a ratioof inside diameter to height of 1:5 to 1:40 and a ratio of height toinside diameter not exceeding 100:1; said extractor having a pluralityof outlets at varying heights, the topmost outlet being disposed nearthe top of said extractor; said washing being carried out bycontinuously feeding substantially 6 to 22 kg./h. of said granules intothe extractor from the top and allowing the compact layer formed toslide downward while continuously feeding water at said temperaturesinto the bottom of said extractor and allowirig it to travel upwardswhile maintaining said temperatures; continuously withdrawing water, nowcontaining dissolved monomeric and oligomeric caprolactam, at saidvarying heights but at all times fully covering said compact layer withwater, in total amounts of substantially 9 to 25 l./h. and containingsaid caprolactam in quantities upto 22 percent; continuously drawing offwashed granules at the bottom of said extractor; and recovering themonomers and oligomers from their solution.

References Cited UNITED STATES PATENTS 2,978,439 4/1961 Kersting 260-783,015,651 1/1962 Kjellmark 26078 3,047,565 7/1962 Braun et al. 260783,149,095 9/1964 Gerutli 260-78 3,245,964 4/1966 Wiemer et a1. 26078WILLIAM H. SHORT, Primary Examiner. H. D. ANDERSON, Assistant Examiner.

